int arr[101];
int count;

void postorder(struct TreeNode* node){
    if(node == NULL) return;

    postorder(node->left);
    postorder(node->right);
    arr[count++] = node->val;
}
int* postorderTraversal(struct TreeNode* root, int* returnSize){
    count = 0;
    postorder(root);
    *returnSize = count;
    return arr;
}

